Currently, the synthesis of polyester resins in the chemical industry is conducted using a titanium-based or tin-based homogeneous catalyst. Because this type of homogeneous catalyst blends completely into the resin, removal of the catalyst by isolation and recovery is difficult, and synthesizing a polyester that contains no residual catalyst is extremely problematic. This residual catalyst promotes reaction between ester bonds and moisture that exists within the polyester resin, causing hydrolysis, and therefore tends to cause a deterioration in the durability and storage stability of the polyester resin. Moreover, this residual catalyst within the polyester resin also tends to participate in the reaction during production of a urethane resin using the generated polyester polyol and an isocyanate, and because the catalyst tends to increase the reactivity, controlling the reactivity becomes difficult. Accordingly, there are currently considerable demands for catalyst-free polyester polyols that contain none of the above type of residual catalyst within the polyester polyol.
Because of the problems outlined above, the amount of a homogeneous catalyst that can be used is usually restricted to a very small amount, and as a result, polyester production requires a considerable length of time. Moreover, because isolation and recovery of the catalyst is problematic, some coloration of the polyester and an effect on the physical properties of the polyester are unavoidable.
Accordingly, a technique has been proposed in which a high-activity organic acid-based catalyst formed from an organometallic compound is used in a small amount to accelerate the esterification reaction (for example, see Patent Document 1). However, in this proposal, because there is still a limit on the amount of catalyst used, a satisfactory amount of the catalyst cannot be applied, meaning a dramatic shortening of the production time cannot be expected.
Furthermore, a technique that uses a solid acid catalyst as a polyester polymerization co-catalyst has also been proposed (see Patent Document 2), but the zirconia molybdate that is used in this patent application has an H0 function of −12.4, and is therefore a so-called “superacid”. However, if this type of solid superacid is used in a dehydration condensation reaction with a glycol and an acid, then because the acid strength is too high, side reactions such as an etherification that proceeds via a glycol dehydration tend to occur, meaning the technique is unsuited to industrial use due to selectivity problems (see example 6 and comparative example 1 of Patent Document 2).
Patent Document 1:
Japanese Unexamined Patent Application, First Publication No. 2005-118714
Patent Document 2:
Japanese Unexamined Patent Application, First Publication No. 2006-265416